The present disclosure relates to a multicore fiber and a manufacturing method of the multicore fiber.
In recent years, in order to increase transmission capacity, optical fibers suitable for space division multiplexing (SDM) systems have been developed. A multicore fiber is an example of such an optical fiber. In the multicore fiber, a plurality of independent cores are arranged in one optical fiber, so that the SDM systems may be implemented. On the other hand, in a multimode fiber capable of propagating signal light in a plurality of propagation modes (hereinafter, simply referred to as “modes”) in a single core, there is a capability of realizing an increase in communication capacity by mode multiplexing transmission (refer to Lars Gruner-Nielsen, et al. “Few Mode Transmission Fiber With Low DGD, Low Mode Coupling, and Low Loss”, J. Lightwave Technol. Vol. 30, No. 23 (2012), pp. 3693-3698.). Herein, since a normal multimode fiber includes a large number of modes, it is difficult to individually control the modes. However, a few-mode fiber where the number of modes is limited to a small number such as about 10 or less has been studied as a new axis of mode multiplexing transmission because all modes may be controlled to be used for propagation (refer to Lars Gruner-Nielsen, et al. “Few Mode Transmission Fiber With Low DGD, Low Mode Coupling, and Low Loss”, J. Lightwave Technol. Vol. 30, No. 23 (2012), pp. 3693-3698.). In addition, at present, few-mode propagating multicore fibers for simultaneously realizing space division multiplexing and mode multiplexing have been reported.
The most serious problem in the case of performing the mode multiplexing transmission is treatment of crosstalk signals generated between the modes. For the crosstalk treatment, a MIMO (Multiple Input, Multiple Output) technology has been utilized, where signal light in each mode is separated to be treated.
However, in the case of performing the mode multiplexing transmission, since group velocities in the optical fiber between modes are different, there is a problem in that the amount of signal processing during the MIMO process is increased. In order to solve the problem, efforts are also made to match the group velocities between modes (for example, a base mode and a higher order mode) in a few-mode fiber.
On the other hand, in the case of performing the SDM systems by using a multicore fiber, even if the refractive index profile of each core is designed to be the same so as to realize the same light propagation characteristics in each core, there occurs a difference between the refractive index profiles which are substantially the same, and thus, a difference in group velocity occurs between the cores. In a case where such a multicore fiber is used, if an MIMO process is performed, there may be a problem in that an amount of signal processing during an MIMO process is increased. In addition, in the case of processing signal light coupled with other cores caused by crosstalk, it is necessary to strictly control the group velocity difference between the cores.
There is a need for a multicore fiber and a method of manufacturing the multicore fiber where a differential group delay between core portions is reduced.